3 Answers
3

One way of approaching this question is to first ask "what would each verse have meant in the mind of the original author" taking account of who we believe was/were the human author's intended audience. When taking this approach, we must also take account of the genre of the writing.

In brief, Proverbs (at least until some of the later sections) are "of Solomon", intended for a general audience of both "naive" and the "wise man", intended to give wisdom to both:

1 The proverbs of Solomon the son of David, king of Israel:2 To know wisdom and instruction,
To discern the sayings of understanding,3 To receive instruction in wise behavior,
Righteousness, justice and equity;4 To give prudence to the naive,
To the youth knowledge and discretion,5 A wise man will hear and increase in learning,
And a man of understanding will acquire wise counsel,6 To understand a proverb and a figure,
The words of the wise and their riddles.NASB

The book was written at a time when there was not a clear picture of exactly what 'eternal life' meant, to the author is very unlikely to have referring to life in that sense - rather a combination of

preventing an early death through foolishness

'life' as in quality of life

eg Proverbs 3:2 :

2 For length of days and years of life
And peace they will add to you. NASB

They are two different types of life. Proverbs speaks mostly to physical things, so the life referred to here is physical life. Living in wisdom keeps us from things that would do us hurt or killed and allows us to have more blessings in this life.

John, however, was talking about the spiritual. Without Christ, we are spirituality dead. With Christ, we have life, hope and peace. Therefore, 1 John 5:12 is speaking to spiritual life.

"Proverbs speaks mostly to physical things"? I find that a rather outlandish claim.
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KazarkMay 26 '12 at 3:50

@Kazark, why? While Proverbs speak to the spiritual, it also dispenses wisdom on money, sex, child-rearing, business transactions, treatment of the poor, education, personal attitudes and many other physical attributes. While you can't truly separate the two (i.e. Spiritual affects and informs the physical and the physical affects and illustrates the spiritual), Proverbs is a practical physical book.
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Andrew NeelyMay 29 '12 at 12:59

Because I would hardly characterize all those things as "the physical". Rather, I might say, "the earthly." And because the whole book is framed in terms of fear of Yahweh (1:7), which is a spiritual consideration.
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KazarkMay 29 '12 at 16:17